How Easter Date Is Determined

The basic rule for determining the date for Easter is that it is on the first Sunday after the first ecclesiastical full moon that occurs on or after March 21st. The beginning date, March 21st, was chosen because it is usually the vernal equinox (generally, the first day of spring). This means that the earliest day of the year that Easter can occur is, of course, March 22nd. Also, because of the regularity of lunar cycles, it can never occur later than April 25th.

Now the ecclesiastical full moon can actually be at a different time than the actual astronomical full moon at least for various locations. This is a result of that confusing confluence of time and space that is known as the international date line. The full moon, at locations near the date-line can occur at the same time on two different days. Be that as it may, the ecclesiastics, perhaps wisely, ignore this anomaly.

The current formula for Easter's date was first developed at the Council of Nicea (convened by Constantine the Great) in 325 A.D. But it was then applied to the Julian Calendar (developed under the Roman Empire). Because there was no leap year to keep the actual year aligned with the calendar year, the date for the vernal equinox would seem to advance at a steady rate through the calendar over the years. Had not Pope Gregory XIII in 1582 executed a reform of the calendar that kept it steadily aligned with the actual astronomical year, Easter, being based on the vernal equinox, would also have lost all consistency of date.

To calculate Easter for any given year between 326 to 4099 AD enter a date: